In the manufacture of optical fiber, a glass preform rod which generally is manufactured in a separate process is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and optical fiber is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower.
Because the surface of the optical fiber is very susceptible to damage caused by abrasion, it becomes necessary to coat the optical fiber, after it is drawn, but before it comes into contact with any surface. Inasmuch as the application of the coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must become solidified rapidly before the optical fiber reaches a capstan. This may be accomplished by photocuring, for example.
Those optical fiber performance properties which are affected most by the coating material are strength and transmission loss. Coating defects which may expose the optical fiber to subsequent damage arise primarily from improper application of the coating material. Defects such as large bubbles or voids, non-concentric coatings with unacceptably thin regions, or intermittent coatings must be avoided. When it is realized that the coating thickness may be as much as the radius of an optical fiber, it becomes apparent that non-concentricity can cause losses in splicing, for example.
Transmission losses may occur in optical fibers because of a mechanism known as microbending. Optical fibers are readily bent when subjected to mechanical stresses, such as those encountered during placement in a cable or when the cabled fiber is exposed to varying temperature environments or mechanical handling. If the stresses placed on the fiber result in a random bending distortion of the fiber axis with periodic components in the millimeter range, light propagating in the fiber core may escape therefrom. These losses, termed microbending losses, may be very large. Accordingly, the fiber must be isolated from stresses which cause microbending. The properties of the fiber coating play a major role in providing this isolation, with coating geometry, modulus and thermal expansion coefficient being the most important factors.
Two types of coating materials are used to overcome this problem. Single coatings, employing a relatively high shear modulus, e.g. 10.sup.9 Pa, or an intermediate modulus, e.g. 10.sup.8 Pa, are used in applications requiring high fiber strengths or in cables which employ buffer tubes where fiber sensitivity to microbending is not a significant problem.
Dual coated optical fibers increasingly are being used to obtain design flexibility and improved performance. A reduction in the modulus of the coating material reduces microbending sensitivity by relieving stress caused in the fiber. Typically, an inner or primary coating layer that comprises a relatively low modulus material, e.g. 10.sup.5 -10.sup.7 Pa, is applied to the optical fiber. The modulus of the primary coating should be effective in promoting long bending periods for the fiber which are outside the microbending range. Such a material reduces microbending losses associated with the cabling, installation or environmental changes during the service life of the optical fiber. In order to meet temperature conditions in expected areas of use, the low modulus coating material must be effective in the range of about -40.degree. to 77.degree. C. An outer or secondary coating layer comprising a relatively high modulus material is applied over the primary layer. The outer coating layer is usually of a higher modulus material to provide abrasion resistance and low friction for the fiber and the primary coating layer. The dual coating serves to cushion the optical fiber by way of the primary layer and to distribute the imposed forces by way of the secondary layer, so as to isolate the optical fiber from bending moments.
One method of applying dual layers of coating materials to a moving optical fiber is disclosed in U.S. Pat. No. 4,474,830 which issued on Oct. 2, 1984, in the name of C. R. Taylor. An advanced system for applying dual coatings on drawn optical fibers is disclosed in application Ser. No. 092,117 which was filed on Sept. 2, 1987 in the names of J. A. Rennell and Carl R. Taylor.
After the coating material or materials have been applied to the moving optical fibers, the coating material or materials are cured, typically by exposure to ultraviolet radiation. In some coating systems, a primary coating material is applied and cured by subjecting it to ultraviolet energy prior to the application of the secondary coating material. If the primary coating material is not maintained at a sufficiently low temperature when the fiber enters apparatus which applied the second coating material, the viscosity of the primary coating material will be so low that variations of the first coating material can result. Such an undesired temperature can occur if an excessive amount of infrared radiation reaches the coating material. In U.S. Pat. No. 4,636,405, this is overcome by surrounding the optical fiber by a chamber that is transparent to ultraviolet light but which includes a jacket through which water flows to absorb the infrared energy.
It is most important that the shear modulus of the coating material on the optical fiber be in a range of desired values. Typically, samples of the coating material which is to be applied to the optical fiber are cured in sheet form, usually at room temperature, for evaluation. Studies have shown that whereas the modulus of a coating material in sheet form is satisfactory, its value exceeds that of the coating material after it has been applied to the optical fiber and cured.
Seemingly, the prior art does not include a coating and curing arrangement which applies coating materials within an applicator at relatively high line speeds, and which is accomplished to control the modulus of the cured coating material. What is sought and what is not provided is a system for applying coating materials to optical fiber in which the cured coating material on the optical fiber has a modulus which falls in a desired range. In order to be able to have assurance regarding the modulus of the coating materials after having been applied to optical fiber, reliance must be had on the test results. Methods need to be provided to insure that the modulus of a curable coating material after it has been applied to the optical fiber and cured will be the same as that determined and found acceptable from test results.